1. Field of the Invention
The present invention relates to an ergonomically designed force sensor system suitable for monitoring weight bearing or temporal parameters on different body regions, such as the foot, knee, and palm. The sensor system may be used in a weight bearing biofeedback system or a functional electrical stimulation system. The foot sensor system may be connected to a portable control unit including a weight bearing program for neurologic, orthopedic or pediatric gait rehabilitation and/or used in connection with a two channel peroneal stimulator for controlling the lower leg muscles to normalize dynamic walking patterns. On the other hand, the knee and palm sensor systems may be used in a weight bearing feedback system in physical stimulation of neurologic and pediatric injuries or for controlling an electronic orthosis.
2. Description of the Prior Art
Stroke is the leading cause of disability in the elderly and a significant source of disability in younger adults. More than 700,000 strokes occur each year, with a prevalence of approximately 3 million. Although stroke is uncommon under the age of 50, the incidence of stroke doubles with each decade after the age of 55. Nearly a third of all stroke survivors will have significant residual disability, with older individuals generally experiencing slower functional recovery. The economic burden associated with stroke is estimated to be more than $30 billion in health care costs and lost productivity each year, making stroke one of the most expensive illnesses in the United States (Chae et al., “Neuromuscular Stimulation for Motor Relearning in Hemiplegia. Critical Reviews in Physical and Rehabilitation Medicine, 11:279-297, 1999).
Hemiparesis due to stroke often results in spastic drop-foot (i.e., the loss of ability to dorsiflex the foot on the affected side). One approach to the management of spastic drop-foot is the prescription of an ankle foot orthosis (AFO), which holds the foot in a neutral position to prevent it from dragging during the swing phase of gait. An alternative approach is active stimulation of the dorsi and plantar flexors.
Electrical stimulation for correction of spastic drop foot in hemiplegia was just applied by Liberson and coworkers in 1961 (Liberson et. al., Functional Electrotherapy, Stimulation of the Peroneal Nerve Synchronized with the Swing Phase of Gait of Hemiplegic Patients. Arch Phys Med Rehab, 42:101-105, 1961). Surface electrodes were applied over the peroneal nerve at the head of the fibula. A stimulator worn around the waist was controlled by a footswitch in the heel of the shoe of the affected limb. When the patient lifted the heel to take a step, the stimulator was activated. Stimulation was stopped when the heel contacted the ground. This system, the peroneal stimulator (PS), produces dorsiflexion and eversion of the foot during the swing phase of gait (Granat et al. Peroneal Stimulator: Evaluation for the Correction of Spastic Drop Foot in Hemiplegia. Arch Phys Med Rehab, 77:19-24, 1996). This system and other electrical stimulation systems are dependent on a sensor system to accurately sense when and to measure how much force is being applied to a region or regions of the foot.
There are a number of insole foot force sensing devices currently used for measuring force on the foot. For example, U.S. Pat. No. 4,745,930 discloses a flexible force sensing insole which incorporates multiple electrical switches which close after a certain threshold level of force is imposed on the insole. U.S. Pat. No. 5,033,291 discloses a force sensing device which uses a plurality of intersecting electrodes. The electrodes act as open circuit switches at each intersection which close when force is applied to the insole at that intersection location. The resistance between the two electrodes varies with the amount of force applied. U.S. Pat. No. 4,426,884 discloses a flexible force sensor which acts as an open circuit, closing with the application of force on the sensor and having resistance that varies with the amount of force.
All of the known foot force measurement devices function to convert mechanical force into a suitable signal medium, usually electrical signals. Consequently, the devices can be conveniently categorized according to the type of sensor used to convert changes in mechanical force to changes in electrical signals. These types of sensors include switches, strain gauge sensors that respond to mechanical deformation, single direct electronic force sensors, multiple direct electronic force sensors with random spacing, and multiple direct electronic force sensors with regular spacing. The sensors which measure mechanical deformation of structural elements supporting the wearer's foot by use of electrical wire or ribbon type strain gauges accurately measure weight, but they are also disadvantageous because of their bulk and weight.
The multiple direct electronic force sensor system taught in U.S. Pat. No. 4,813,436 to Au measures forces only where the individual sensors are attached to the foot. If the measurements are used to compute total force applied to the foot and are variously spaced, the contributory area of each sensor must be used in the necessary computation of the total force applied. This system is disadvantageous in that the relative position of each sensor must be separately determined for each person on which the sensor is used. This problem is solved by the multiple direct electronic force sensors taught in U.S. Pat. Nos. 4,734,034, 4,856,993 and 5,033,291 to Maness et al. in which the sensors are regularly spaced. Since the relative position of each sensor is fixed, a mathematical position of the location of each sensor can easily be made to be part of a permanent computer database. These sensor arrays are very thin and very light weight, but they cannot conform to a compound curved surface without wrinkling. Such wrinkled or folded thin film sensor arrays will produce erroneous results. For example, if the sensor array is folded so that two separate sensors are positioned one above the other, they both measure the same force. This is an obvious error. A folded sensor array also may produce an electrical signal from the folding alone, another obvious error. Folding or wrinkling also subjects the sensor array to severe fatigue stress, which can lead to early and sudden failure.
U.S. Pat. No. 3,881,496 issued to Vrendenbregt et al. discloses an apparatus and method for electrically stimulating leg muscles using an air-filled chamber located in the sole of the shoe beneath the ball of the foot. The chamber is coupled through an air channel or a thin hose and a diaphragma to a microswitch located in the heel. The switch activates an electric pulse generator in synchronism with the normal walking pattern.
U.S. Pat. No. 3,974,491 issued to Sipe discloses a sensor having a fluid filled chamber that is a continuous, resilient tube having a circular cross section. The tube is coiled under the heel and the sole of a patient's foot inside a sponge rubber footpad. The footpad is between adhesive sheets of flexible, dimensionally stable material such as rubber-coated fabric. The foot pad does not measure the total load placed on the limb because a portion of that load is done by a sponge rubber pad and because the tube is not directly beneath all parts of the foot.
U.S. Pat. No. 5,107,854 issued to Knotts et al. discloses a single fluid filled plantar chamber that supports the entire load borne by a patient's foot. The plantar chamber is connected to a remote pressure sensing device that is responsive to pressure changes transmitted by the single fluid filled plantar chamber. The sensing device disclosed by Knotts et al. provides an accurate measurement of the force on the foot because the remote pressure sensor is not positioned in the insole, and, therefore, is not subject to the problem of electrical contact failure
While the sensing device disclosed by Knotts et al. provides an accurate measurement of the force on the foot, it comprises only a single chamber that is used to provide a single force measurement. In the course of rehabilitating the foot, however, it is often desirable to obtain force measurements from a plurality of locations on the foot such as, for example, the heel region and the toe region. U.S. Pat. No. 3,791,375 issued to Pfeiffer discloses a remote displacement measuring device that is connected to two units, a heel unit and a toe unit, located in the insole. The units deflect and change their volume in accordance with the amount of load placed thereon. The displacement measuring device is signaled with an electrical alarm to indicate when a predetermined load on the units is reached. The displacement measuring device consists of a single sensor such as, for example, a bellows that measures the combined total displacement from both the heel and the toe unit.
While the sensing device disclosed by Pfeiffer provides an accurate single measurement of two regions of the foot, it comprises only a single sensing device that is used to obtain a cumulative single measurement. In addition to measuring the cumulative force on a plurality of regions of the foot, it is also desirable to obtain and compare a plurality of measurements, each from a different location of the foot. Additionally, it is desirable to obtain such measurements using sensors that are remote from the insole. Sensors within an insole are subject to the problem of electrical contact failure, and an awkward placement or posture of the foot may result in a failure to activate insole sensors. The electronic components in existing insoles increase the size of the insole, causing blisters and skin irritation and often forcing the patient to purchase increased sized shoes big enough to fit the bulky insole. Thus, there is a need in the art for a force sensing system that uses a plurality of remote sensors to obtain and compare measurements from a plurality of regions.